On the other hand, you don't need to know it all. It is not possible and it is not neccessary.
In some cases, you want to go deep, when in other cases, you want to go wide. In some cases, you need to know how to do it with certain tools, when in other cases, all you need to know is whether it is doable with certain tools.
It is also important to know where to find answers, might be a book, a manual, a web site etc. In this way, you know how to turn "unknown" into "known". That's one of the most imprtant area of knowledge you really need.